Air drilling is a downhole drilling operation that relies on compressed air or gases, delivered through the inside of a drill string to power a drilling motor and cause a drill bit to rotate or to power an air drilling hammer, and to cool the drill bit at the terminal end of the drill string and transport rock cuttings in the borehole to the surface. It is especially useful in drilling operations concerning dry hard rock lithology. At the terminal end of the drill string is a bottom-hole-assembly (BHA) which comprises a drill bit, a downhole motor of a rotary steerable system, sensors of survey equipment for Logging While Drilling (LWD) and/or Measurement-while-drilling (MWD) to evaluate downhole conditions as well depth progresses, means for transmitting telemetry data to surface, and other control equipment. Compared to mud drilling for hard rock lithology operations, air drilling is more economical, cleaner in terms of operating conditions, more effective in controlling pressure especially through lost circulation zones, and generally has faster rates of penetration. Bidirectional data communication between the telemetry and control interface at the surface of the drilling operation and points along the drill string (the terminal end) is used for MWD and LWD and for controlling the drilling operation beneath the surface. There are various known ways of implementing bidirectional data communication including wireless means such as electromagnetic (EM) telemetry and wired means such as the IntelliServ™ system by National Oil Varco. EM telemetry has found utility in specific applications such as underbalanced drilling or air drilling. However, this telemetry technique transfers data at a relatively low rate, is generally expensive and time consuming to deploy, and is not suitable for certain types of formations. Wired telemetry has also been used to facilitate bidirectional data communication in drilling operations. However, wired telemetry also has limitations such as vulnerability to damage.
Microwave frequency telemetry transmission has been proposed for air drilling applications to facilitate bidirectional data communication between the surface and terminal ends of the drill string. Using the drill string as a circular waveguide, transmitting telemetry and control instructions via microwave frequencies has the potential to provide a relatively high rate of wireless data transfer. However, this telemetry technique currently faces a number of challenges in downhole drilling operations, especially air drilling operations. A typical drill string has a number of obstructions which can attenuate and reflect microwave signals and thus is not an ideal microwave waveguide. Signal interference and degradation can be especially pronounced in long drill strings, some of which can be up to 10,000 feet in length. Air drilling drill strings pose particular challenges as one way valves along the drill string, compressors and boosters at surface and components of the bottom hole assembly all present significant reflection and attenuation challenges. Further, during an air drilling operation, foam and mist can be injected into the drill string to improve air drilling functions, but tend to impede or interrupt microwave communication along all or part of the drill string.